Eager to get its 175,000-square-foot expansion back on track, the Peabody Essex Museum has selected Ennead Architects to design the project. The New York firm comes fresh off its acclaimed expansion and renovation of the Yale University Art Gallery.

Ennead replaces Rick Mather Associates, the small London-based firm PEM had originally hired to do the job. Mather’s illness less than a year after expansion plans were announced and his death earlier this year were factors in PEM’s completion schedule being pushed back three years, and led to the museum cutting ties and recruiting a handful of other firms, including Ennead, to apply to take over the design. The expansion will add galleries, education spaces, and a new restaurant.

When announcing the decision to move on from Mather’s firm in May, PEM director Dan Monroe said the firm’s size — just 15 architects on staff — and Mather’s intense involvement in the project made it hard to proceed after his death.

Ennead has about 125 staff architects and has worked on a range of museum projects, including the National History Museum of Utah, the Brooklyn Museum, and the William J. Clinton Presidential Center in Little Rock.

Working with a larger firm “means you’re not dependent on only one or two people in terms of a design team,” said Monroe.

He and the museum’s board were also impressed by Ennead’s Yale project, Monroe said, in which buildings from different eras were unified as part of a $135 million expansion that stretched over 14 years. That project drew raves upon its completion in December 2012.

“There are many similarities,” between the New Haven and Salem museum challenges, said Richard Olcott, a design partner at Ennead. “This project has landmark buildings as well. It couldn’t possibly be a more diverse collection of architecture. That is going to require some delicacy, some finesse, and not a heavy hand, a light one.”

Suzanne Kreiter/Globe Staff/file 2011

Of working with a larger firm, Peabody Essex Museum director Dan Monroe said, “You’re not dependent on only one or two people in terms of a design team.”

The expansion was originally meant to be finished in 2016, but an initial delay and the decision to move on from Mather’s firm have pushed that out to 2019.

In discussing Ennead, Monroe said there were several factors beyond the firm’s size, or as he calls it, “bench strength,” that made the firm ideal at this point. He appreciates Ennead’s approach in what he says is creating spaces that stress function over architectural flash.

“There are a number of well-known architects who have a signature style which they replicate with variations in the work they do,” said Monroe. “That’s fine. It’s just not what we’re interested in doing. We want somebody who works with us, brings design solutions to the table that are not prefaced or based upon an existing set of architectural moves.”

He also said Ennead uses miniature models of designs, which he appreciates.

“That might seem like an odd criterion,” said Monroe, “but a great deal of design today is done on computer. It’s all great, but in terms of actually working out what a building is going to look and feel like, how it’s going to function, we think it’s really essential a firm make extensive use of models.”

Olcott said Ennead is excited to dig into the project. The only reason the firm wasn’t involved in the initial proposals that led to Mather’s selection was “they didn’t ask,” he said. In describing the challenges ahead, Olcott talked of the many elements on the Salem campus, from East India Marine Hall, finished in 1825, to the 2003 expansion designed by Moshe Safdie.

“It’s very daunting but that’s what makes it interesting,” he said. “We thrive for that kind of project. It would be very boring if it was just some green field somewhere. There’s so much to draw from there that can make you think creatively and differently. It really is an unbelievably rich and dense environment.”